An unsolved mystery of development concerns how growth and form are controlled. The regulation of cell proliferation is central to this problem, and the mechanisms controlling cell cycle arrest upon cell differentiation are particularly relevant. Cell cycle exit is also critical in carcinogenesis, where it is bypassed, and in wound healing and regeneration, where it is reversed to allow renewed proliferation. While cell cycle exit has been studied in cell culture and in vivo in several model organisms, the mechanisms that couple differentiation signals to the cell cycle control apparatus remain poorly understood. No general paradigm exists explaining the ubiquitous coupling of cell differentiation to G1 arrest. This proposal is to study cell cycle exit at differentiation in Drosophila in three well-characterized contexts: the embryo, the wing, and the eye. We will test the hypothesis that differentiation signals dominantly suppress the transcription of cell cycle control genes via an E2F/RB-independent mechanism. In Aim 1, we manipulate the activity of known cell cycle control genes to define the mechanism that triggers cell cycle exit, and we also analyze changes in gene expression during the exit process. In Aim 2, we study the transcriptional regulatory region of the critical cell cycle control gene, cyclin E, to identify cis-acting elements, and eventually trans-acting factors, that mediate its silencing at differentiation. In Aim 3 we perform genetic screens in the fly to identify novel genes that are functionally important for cell cycle exit. The combined results should distinguish between the many plausible explanations of cell cycle exit suggested in the literature, identify new gene products that mediate cell cycle exit, and provide a working paradigm for future studies of how differentiation signals interface with the cell cycle control apparatus. Because the genetic networks that orchestrate tissue patterning, cell differentiation, and cell cycle control are conserved between Drosophila and man, the results obtained herein should inform ongoing studies of human development and disease.